You should not be editing a cdata table. __cdata tables are temporary tables that routinely get dropped by the system and regenerated as needed.
You haven't told us anything about your environment (webserver, php version, osticket version, etc).
If you are running a contemporary version of osTicket (1.12.x) then you just go to Admin panel -> Settings -> Tickets -> Queues and add the column that you want to add.